Optical fiber based distributed acoustic sensors (DAS) are known in the art. One high performance example is the iDAS™, available from Silixa Limited, of Elstree, UK. Further details of the operation of a suitable DAS are given in WO2010/0136809 and WO2010/136810, which also disclose that distributed acoustic sensors may be used for in-well applications, in that the acoustic noise profile can be used to measure the flow by noise logging at every location along the well. In addition, the noise spectrum can be used to identify the phase of the fluid.
Flow monitoring in wells, pipes, or other fluid carrying structures using eddy tracking is a technique that shows some promise. A DAS system is typically used to detect the noise generated by an eddy in the fluid carrying structure and to track the noise along the structure as the eddy moves with the flow. From this tracking, knowledge of the flow velocity can be obtained.
A typical eddy lifetime is usually long enough for the eddy to move along the structure with the flow for two or three diameters of the structure i.e. two to three pipe diameters in the case of a pipe. However, the typical native sensing resolution of an optical fiber based DAS is around 5 m, although the best in class sensors, such as the iDAS™ sensor available from Silixa Limited, of Elstree, UK can achieve sensing resolutions of around 1 m along lengths of fiber up to 50 km in length. Even with such high spatial sensing resolution, however, in typical pipes the eddy lifetime is shorter than the highest spatial sensing resolution when the optical fiber is deployed linearly along the pipe or structure to be monitored.
To try and get around this issue, in WO2010/136810 we described how an optical fiber might be helically wrapped around a section of pipe or a well. This results in there being a greater length of fiber in the particular section of the pipe around which the fiber is wrapped, and hence increases the spatial sensitivity of the DAS system in that section. For example, for a 30 cm pipe with a 1 m native sensing resolution, in the helically wrapped section the DAS would be able to make a discrete acoustic measurement for approximately every turn of fiber. The longitudinal sensing resolution along the pipe is therefore vastly increased.
However, wrapping fiber around a pipe or other fluid carrying vessel as described is costly, and depending on the environment and installation conditions, may not be technically or economically feasible. In addition, once installed, such sections of wrapped fiber can be difficult to remove intact, requiring unwinding from around the pipe. A more user friendly and economic way of providing high density sections of fiber for pipe or other fluid carrying vessel monitoring is therefore required.
WO2008/011058 describes a fiber-optic mat sensor wherein optical fiber is distributed about a support structure such as a mat fabricated from appropriate flexible material. The distributed optical fiber includes multiple fiber optic bends, which in one embodiment follow a circuitous route. The mat sensor is intended for use as a baby monitor, to detect baby movements. However, no detailed disclosure is provided as to precisely how the fiber is distributed across the mat, for example so as to minimise fiber bending losses, or to improve sensitivity in particular areas of the mat. Several issues therefore remain to be resolved to provide a practical, optimised fiber-optic mat sensor.